Conventionally, there is known a power supply apparatus which supplies electric power to a load (see, e.g., Japanese Patent Application Publication No. 2002-354783). In the power supply apparatus disclosed in Japanese Patent Application Publication No. 2002-354783, an AC voltage of a commercial AC power source is full-wave rectified by a rectifier having a diode bridge, and a ripple voltage outputted from the rectifier is converted by a step-up/step-down chopper circuit having two transistors. Further, a DC voltage outputted from the step-up/step-down chopper circuit is converted into a high frequency power by an inverter circuit, and the high frequency power outputted from the inverter circuit is supplied to a load circuit.
The step-up/step-down chopper circuit includes a step-down chopper on a front side and a step-up chopper on a rear side. A common inductor is used in both of the step-down chopper and the step-up chopper. A switching element of the step-down chopper is switched on and off by a PWM control circuit through a driver in synchronism with a switching element of the step-up chopper. In the step-up/step-down chopper circuit, the inductor is used for both step-up and step-down. Accordingly, an input voltage can be not only increased but also decreased.
However, in the power supply apparatus described above, when the switching element of the step-down chopper is short-circuited, the voltage outputted from the rectifier is applied to the load circuit without being decreased, resulting in an excessive current (over-current) flowing through the load circuit. For example, in a case where the power supply apparatus is used as an LED lighting device having a light source unit having light emitting diodes as a load, if an over-current flows through the light source unit, the lifetime of the light source unit may be shortened and the light source unit can be broken.